mysql表连接选择

SELECT id, topic 
  FROM topics 
 WHERE id IN (SELECT id_topic 
                FROM answers 
                WHERE id_user = <USERID>)

SELECT id, topic 
  FROM topics 
 WHERE EXISTS (SELECT * 
                 FROM answers 
                WHERE id_user = topics.id)

SELECT * FROM topics
WHERE id IN (SELECT id_topic FROM answers WHERE id_user = ?);

子查询是否多次返回某些值并不重要。外部查询中的任何给定行都将匹配或不匹配。

正如E.F.Codd所说,“如果某事是真的,说两次并不能使它更真实。”

如果您想查看用户回答了哪些主题,请尝试以下操作:

SELECT t.title, a.id_user, COUNT(a.id)
FROM topics t LEFT JOIN answers a
  ON (t.id=a.topic_id)
GROUP BY t.title, a.id_user;

JOIN LEFT JOIN .